Approximately 70% of breast

Josh Lauring and Antonio C. Wolff Evolving Role of the Estrogen Receptor as a Predictive Biomarker: ESR1 Mutational Status and Endocrine Resistance in Breast Cancer (J Clin Oncol 2016;34(25):2950 2952.) Approximately 70% of breast cancers express the estrogen receptor alpha (ER), and endocrine therapies targeting ER are a cornerstone of their treatment. Unfortunately, a subset of patients treated with adjuvant endocrine therapy will develop metastatic disease, and patients treated with endocrine therapies in the metastatic setting will, in time, confront tumor endocrine resistance. Many mechanisms of endocrine resistance have been identified in preclinical models and clinical studies, including growth factor receptor signaling pathways (such as human epidermal growth factor receptor 2) and alterations in transcriptional coregulators. 1 An activating mutation in ESR1, the gene encoding ER, was first described in a metastatic breast cancer in 1997. 2 However, subsequent studies performed in primary breast cancers did not identify frequent ESR1 mutations, and the potential clinical significance of ESR1 mutations was underappreciated. It was not until 2013 that a series of studies using next-generation DNA sequencing renewed interest by demonstrating a high prevalence (11% to 55%) of ESR1 mutations in metastatic ER-positive breast cancers with prior aromatase inhibitor (AI) therapy, but not in primary breast cancers (<1%). 3 8 Most mutations occur in hotspots in the ligand-binding domain and result in constitutive, ligand-independent activity of ER, explaining how these mutations seem to be selected by the low-estrogen environment with AI therapy. Preclinical studies have shown reduced sensitivity of mutant ERs to tamoxifen, 2,5 7 suggesting that tamoxifen might only have efficacy against ESR1 mutant tumors at higher-than-standard dosages, if at all. The selective ER degrader fulvestrant seems to retain activity against mutant ERs, albeit perhaps with reduced potency, as does the cyclin-dependent kinase-4/6 inhibitor palbociclib. 3 7,9 Altogether, these data suggest that ESR1 mutations could be a major clinical mechanism of endocrine resistance. Therefore, the clinical responsiveness of ESR1 mutant breast cancers to various endocrine therapies is now a question of great interest. Numerous recent reports have demonstrated the detection of mutant DNA alleles as tumor-specific biomarkers in cell-free DNA (cfdna) from blood. 10 13 Droplet digital polymerase chain reaction (ddpcr) is a highly sensitive and specific technique that has been used for ESR1 mutation detection in plasma. 14 18 A retrospective single-institution analysis documented that ESR1 mutations detected in plasma cfdna by ddpcr were associated with a lack of response to subsequent AI therapy. 15 However, prospective data have been lacking, and the clinical impact of alternative endocrine therapies, including fulvestrant, has not been examined. In the report that accompanies this editorial in Journal of Clinical Oncology, Approximately 70% of breast cancers express the estrogen receptor alpha (ER), and endocrine therapies targeting ER are a cornerstone of their treatment. Many mechanisms of endocrine resistance have been identified in preclinical models and clinical studies, including growth factor receptor signaling pathways (such as human epidermal growth factor receptor 2) and alterations in transcriptional coregulators. Most mutations occur in hotspots in the ligandbinding domain and result in constitutive, ligandindependent activity of ER, explaining how these mutations seem to be selected by the lowestrogen environment with AI therapy. Preclinical studies have shown reduced sensitivity of mutant ERs to tamoxifen, suggesting that tamoxifen might only have efficacy against ESR1 mutant tumors at higherthan-standard dosages. Data suggest that ESR1 mutations could be a major clinical mechanism of endocrine resistance. 43

Syllabus In the Study of Faslodex With or Without Concomitant Arimidex Versus Exem estane Following Progression on Non-Steroidal Aromatase Inhibitors (SoFEA) trial, women whose cancer had progressed after a period of sensitivity to NSAI. Plasma samples were available for 521 of the PALOMA-3 patients (69.1%), but only 162 of the SoFEA patients (22.4%) because most were lost in a fire. The investigators analyzed seven ESR1 mutations using multiplex ddpcr, covering nearly all mutations described to date. Mutation status was analyzed as a binary outcome, with a positive mutation call requiring a minimum of two positive droplets in a minimum of 0.5 ml (PALOMA-3) or 1 ml (SoFEA) of plasma. Patients in SoFEA with ESR1 mutations (39.1%, of which 49.1% were polyclonal) had improved progression-free survival (PFS) receiving fulvestrant (n = 45) compared with exemestane (n = 18; hazard ratio [HR], 0.52; 95% CI, 0.30 to 0.92; P =.02). Fulvestrant plus palbociclib improved PFS compared with fulvestrant plus placebo in patients with ESR1 mutant. In multivariable analysis, ESR1 mutations were associated with AI exposure, sensitivity to prior endocrine therapy, and bone or visceral disease. Fribbens et al 19 performed a prospectiveretrospective analysis of ESR1 mutations in baseline plasma samples from two randomized phase III clinical trials comparing different endocrine therapies for metastatic ER-positive breast cancer after nonsteroidal AI (NSAI). In the Study of Faslodex With or Without Concomitant Arimidex Versus Exemestane Following Progression on Non-Steroidal Aromatase Inhibitors (SoFEA) trial, women whose cancer had progressed after a period of sensitivity to NSAI, defined as recurrence after at least 12 months of adjuvant NSAI or disease progression after at least 6 months of first-line metastatic treatment with an NSAI, were randomly assigned to receive the steroidal AI exemestane, fulvestrant 250 mg, or the combination of the NSAI anastrozole and fulvestrant 250 mg. 20 In the Palbociclib Ongoing Trials in the Management of Breast Cancer (PALOMA)-3 trial, women whose cancer had recurred within 12 months of completion of adjuvant endocrine therapy or had disease progression during palliative endocrine therapy were randomly assigned to receive fulvestrant 500 mg plus placebo versus fulvestrant 500 mg plus palbociclib. 21 Plasma samples were available for 521 of the PALOMA-3 patients (69.1%), but only 162 of the SoFEA patients (22.4%) because most were lost in a fire. The investigators analyzed seven ESR1 mutations using multiplex ddpcr, covering nearly all mutations described to date. Mutation status was analyzed as a binary outcome, with a positive mutation call requiring a minimum of two positive droplets in a minimum of 0.5 ml (PALOMA-3) or 1 ml (SoFEA) of plasma. Patients in SoFEA with ESR1 mutations (39.1%, of which 49.1% were polyclonal) had improved progression-free survival (PFS) receiving fulvestrant (n = 45) compared with exemestane (n = 18; hazard ratio [HR], 0.52; 95% CI, 0.30 to 0.92; P =.02), whereas patients with wild-type ESR1 had similar PFS receiving either treatment (HR, 1.07; 95% CI, 0.68 to 1.67; P =.77). In PALOMA-3, ESR1 mutations were found in the plasma of 25.3% (91/360) of patients, of which 28.6% (26/91) were polyclonal. Fulvestrant plus palbociclib improved PFS compared with fulvestrant plus placebo in patients with ESR1 mutant (HR, 0.43; 95% CI, 0.25 to 0.7; P =.002) and ESR1 wild-type (HR, 0.4; 95% CI, 0.35 to 0.70; P <.001) tumors. In multivariable analysis, ESR1 mutations were associated with AI exposure, sensitivity to prior endocrine therapy, and bone or visceral disease. Many women with metastatic ER-positive/ human epidermal growth factor receptor 2-negative breast cancer now receive letrozole and palbociclib as first-line endocrine therapy. 22 Although Fribbens et al 19 showed encouraging activity of both fulvestrant and palbociclib against ESR1 mutant cancers, we do not know how palbociclib will affect the emergence of ESR1 mutations or whether fulvestrant plus palbociclib will have the same benefit in patients with prior palbociclib exposure. Although PFS with fulvestrant plus palbociclib was similar in patients with ESR1 mutant or wild-type tumors, this analysis lacked power to conclude that fulvestrant alone or in combination with palbociclib fully overcomes the adverse prognosis of ESR1 mutations. This work highlights the potential advantages of liquid biopsy as a noninvasive tool to monitor the emergence of ESR1 mutations and their response to treatment. The high frequency of polyclonal ESR1 mutations found in this study and others 14,15,18,23 suggests that a metastatic tissue biopsy from a single site would fail to capture the complexity of resistant disease. To date, studies to detect ESR1 mutations in cfdna have been conducted mostly in individual research laboratories using variable sample preparation methods, primer/probe combinations, and detection cutoffs. There is currently no agreed-upon standard for quantifying mutant tumor alleles in cfdna as an absolute number, as 44

Evolving Role of the Estrogen Receptor as a Predictive Biomarker a concentration relative to cfdna genome equivalents, or as a concentration per volume of plasma. Total cfdna quantity in a sample will vary, depending on location and burden of metastases and whether handling and processing of blood to plasma resulted in lysis of white blood cells, a source of excess wild-type DNA alleles. Use of special blood collection tubes and handling reduces such contamination by orders of magnitude and improves rare mutation detection. 24 Thus, the reporting by Fribbens et al 19 that archived plasma samples processed using routine protocols could be analyzed successfully for ESR1 mutations is a potentially significant demonstration that could open up archived samples from many pivotal, randomized phase III trials to similar prospective-retrospective analysis of this and other cfdna biomarkers. However, the assay analytical issues described previously will require careful attention when comparing results across studies. In addition to assay analytical validation, a clinically useful ESR1 cfdna assay will also need to meet standards of clinical validity and clinical utility by helping the clinician select a more effective therapy or avoid an ineffective one. 25 These data raise several questions regarding such an assay. Is a binary determination of ESR1 mutation status as used by Fribbens et al 19 the most clinically useful one? Will there be a continuum of clinical outcomes on the basis of the mutant allelic fraction in the cancer or mutant allele kinetics? Is the specific mutation important? Preclinical studies have shown differences among ESR1 mutations in terms of sensitivity to endocrine therapies, 5 but clinical analyses have been underpowered and have not yet reached a consensus on this issue. 19,23,26 Consequently, the clear answer to the important question of whether ESR1 mutation testing is ready for immediate clinical use is an unequivocal not yet. The available data show that ESR1 mutant tumors in patients with clinical progression who are taking an NSAI do not benefit from exemestane. Clinical trials have reported varying degrees of benefit to exemestane after NSAI, perhaps because of differences in patient characteristics. 27,28 And, even without an ESR1 mutation test, many clinicians would be inclined to treat such a patient with fulvestrant alone or with palbociclib, 21 or with exemestane and everolimus. 28 Still, the analysis of SoFEA by Fribbens et al 19 does suggest that patients with ESR1 wild-type tumors and disease progression after initial clinical sensitivity to an NSAI might benefit from exemestane, whereas patients with ESR1 mutated tumors might be better served by fulvestrant or fulvestrant plus palbociclib. If these observations are independently confirmed, lack of ESR1 mutation could then be used to select those patients who would be treated with exemestane alone before moving on to fulvestrant, much as KRAS wild-type status is required for clinical benefit from anti epidermal growth factor receptor antibody therapy in colon cancer. 29 These are still early days in our understanding of the biology of ESR1 mutations in breast cancer. ESR1 mutations have been detected at a much higher frequency in patients who received AI for metastatic disease than in those who only received adjuvant AI. 15 Schiavon et al 15 explained this difference by hypothesizing that preexisting ESR1 mutant subclones are selected by AI therapy, but that the tumor burden in the micrometastatic setting may be too low for many such clones to be present. ESR1 mutations have been identified only rarely in patients whose sole endocrine therapy was tamoxifen (1/49 in the PALOMA-3 analysis); 7,15,19 however, the duration and setting (adjuvant vs. metastatic) of tamoxifen therapy for these patients has not been reported. Therefore, it is premature to discount the possibility that tamoxifen will select for ESR1 mutations as well. Also much needed are clinical data on the sensitivity of ESR1 mutant breast cancers to tamoxifen, which may be particularly relevant for countries where A clinically useful ESR1 cfdna assay will also need to meet standards of clinical validity and clinical utility by helping the clinician select a more effective therapy or avoid an ineffective one. Preclinical studies have shown differences among ESR1 mutations in terms of sensitivity to endocrine therapies, but clinical analyses have been underpowered and have not yet reached a consensus on this issue. The available data show that ESR1 mutant tumors in patients with clinical progression who are taking an NSAI do not benefit from exemestane. Clinical trials have reported varying degrees of benefit to exemestane after NSAI, perhaps because of differences in patient characteristics. Observations are independently confirmed, lack of ESR1 mutation could then be used to select those patients who would be treated with exemestane alone before moving on to fulvestrant. Much as KRAS wild-type status is required for clinical benefit from anti epidermal growth factor receptor antibody therapy in colon cancer. ESR1 mutations have been detected at a much higher frequency in patients who received AI for metastatic disease than in those who only received adjuvant AI. 45

Syllabus The observations by Fribbens et al will now support studies on earlier stages of disease. Data by Fribbens and others in the last few years now show that mutations in ESR1 might serve as a predictor of resistance to certain endocrine therapies. However, the tortuous history of ER testing need not be repeated. access to fulvestrant, everolimus, and palbociclib is limited. The observations by Fribbens et al 19 will now support studies on earlier stages of disease. For instance, will detection of ESR1 mutations during adjuvant AI therapy affect decisions regarding therapy duration and/or a switch to an alternative adjuvant endocrine therapy? The findings now reported support the design of additional prospective-retrospective studies using plasma repositories from completed adjuvant trials of endocrine therapy. Should ESR1 mutations be detected in these studies, an immediate question of interest will be whether combination or sequential adjuvant strategies might help circumvent their emergence. In 1973, McGuire 30 asserted that detection of ER in breast cancer might serve as a predictor of response to endocrine therapy. Data by Fribbens and others in the last few years now show that mutations in ESR1 might serve as a predictor of resistance to certain endocrine therapies. However, the tortuous history of ER testing need not be repeated, 31 and assay methodologies to detect and quantify ESR1 mutations in blood and tissue must now be standardized. This will be a critical step to allow ESR1 mutational status to be used as an integral biomarker in trials on ER-positive disease and to be tested prospectively as a stratification factor, as an enrichment strategy, and as a therapeutic target in the development of new strategies to overcome endocrine resistance in breast cancer. References 1. 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